Implement 'q_sort

Implement 'q_sort'
function using 'merge'
function.

Change-Id: I2fcf5f88442f68c00e7f92982fa5d5c0cf60895a

Author: PoChuan994

queue.c

@@ -237,8 +237,50 @@ void q_reverseK(struct list_head *head, int k)
     q_restruct(head);
 }
 
+/* meger each elements of queue in ascending/descending order */
+void merge(struct list_head *head,
+           struct list_head *left,
+           struct list_head *right,
+           bool descend)
+{
+    while (!list_empty(left) && !list_empty(right)) {
+        const element_t *l = list_entry(left->next, element_t, list);
+        const element_t *r = list_entry(right->next, element_t, list);
+        if (((descend * 2) - 1) * strcmp(l->value, r->value) > 0)
+            list_move_tail(left->next, head);
+        else
+            list_move_tail(right->next, head);
+    }
+    if (list_empty(head))
+        list_splice_tail(right, head);
+    else
+        list_splice_tail(left, head);
+}
+
 /* Sort elements of queue in ascending/descending order */
-void q_sort(struct list_head *head, bool descend) {}
+void q_sort(struct list_head *head, bool descend)
+{
+    if (!head || list_empty(head) || list_is_singular(head))
+        return;
+
+    struct list_head *slow = head->next, *fast = head->next->next;
+    while (fast != head && fast->next != head) {
+        slow = slow->next;
+        fast = fast->next->next;
+    }
+
+    struct list_head l, r;
+    INIT_LIST_HEAD(&l);
+    INIT_LIST_HEAD(&r);
+
+    list_cut_position(&l, head, slow);
+    list_splice_init(head, &r);
+
+    q_sort(&l, descend);
+    q_sort(&r, descend);
+
+    merge(head, &l, &r, descend);
+}
 
 /* Remove every node which has a node with a strictly less value anywhere to
  * the right side of it */